Hinte Florian, Müller Jendrik, Brune Wolfram
Heinrich Pette Institute, Leibniz Institute for Experimental Virology, Hamburg, Germany.
Heinrich Pette Institute, Leibniz Institute for Experimental Virology, Hamburg, Germany
J Virol. 2021 Mar 25;95(8). doi: 10.1128/JVI.01990-20. Epub 2021 Jan 20.
The unfolded protein response (UPR) and endoplasmic reticulum (ER)-associated degradation (ERAD) are two essential components of the quality control system for proteins in the secretory pathway. When unfolded proteins accumulate in the ER, UPR sensors such as IRE1 induce the expression of ERAD genes, thereby increasing protein export from the ER to the cytosol and subsequent degradation by the proteasome. Conversely, IRE1 itself is an ERAD substrate, indicating that the UPR and ERAD regulate each other. Viruses are intracellular parasites that exploit the host cell for their own benefit. Cytomegaloviruses selectively modulate the UPR to take advantage of beneficial and inhibit detrimental effects on viral replication. We have previously shown that murine and human cytomegaloviruses express homologous proteins (M50 and UL50, respectively) that dampen the UPR at late times post infection by inducing IRE1 degradation. However, the degradation mechanism has remained uncertain. Here we show that the cytomegalovirus M50 protein mediates IRE1 degradation by the proteasome. M50-dependent IRE1 degradation can be blocked by pharmacological inhibition of p97/VCP or by genetic ablation of SEL1L, both of which are components of the ERAD machinery. SEL1L acts as a cofactor of the E3 ubiquitin ligase HRD1, while p97/VCP is responsible for the extraction of ubiquitylated proteins from the ER to the cytosol. We further show that M50 facilitates the IRE1-SEL1L interaction by binding to both, IRE1 and SEL1L. These results indicate that the viral M50 protein dampens the UPR by tethering IRE1 to SEL1L, thereby promoting its degradation by the ERAD machinery. Viruses infect cells of their host and force them to produce virus progeny. This can impose stress on the host cell and activate counter-regulatory mechanisms. Protein overload in the endoplasmic reticulum (ER) leads to ER stress and triggers the unfolded protein response, which in turn upregulates protein folding and increases the degradation of proteins in the ER. Previous work has shown that cytomegaloviruses interfere with the unfolded protein response by degrading the sensor molecule IRE1. Herein we demonstrate how the cytomegalovirus M50 protein exploits the ER-associated degradation machinery to dispose of IRE1. Degradation of IRE1 curbs the unfolded protein response and helps the virus to increase the synthesis of its own proteins and the production of virus progeny.
未折叠蛋白反应（UPR）和内质网（ER）相关降解（ERAD）是分泌途径中蛋白质质量控制系统的两个重要组成部分。当未折叠蛋白在内质网中积累时，诸如IRE1等UPR传感器会诱导ERAD基因的表达，从而增加蛋白质从内质网输出到细胞质，并随后被蛋白酶体降解。相反，IRE1本身是一种ERAD底物，这表明UPR和ERAD相互调节。病毒是细胞内寄生虫，会利用宿主细胞为自身谋利。巨细胞病毒会选择性地调节UPR，以利用其对病毒复制的有益影响并抑制有害影响。我们之前已经表明，鼠巨细胞病毒和人巨细胞病毒分别表达同源蛋白（分别为M50和UL50），这些蛋白在感染后期通过诱导IRE1降解来抑制UPR。然而，其降解机制仍不确定。在这里，我们表明巨细胞病毒M50蛋白通过蛋白酶体介导IRE1的降解。p97/VCP的药理学抑制或SEL1L的基因敲除均可阻断M50依赖性IRE1降解，这两者都是ERAD机制的组成部分。SEL1L作为E3泛素连接酶HRD1的辅助因子，而p97/VCP负责将泛素化蛋白从内质网提取到细胞质中。我们进一步表明，M50通过与IRE1和SEL1L两者结合来促进IRE1-SEL1L相互作用。这些结果表明，病毒M50蛋白通过将IRE1与SEL1L拴系在一起，从而促进其被ERAD机制降解，进而抑制UPR。病毒感染宿主细胞并迫使它们产生病毒后代。这会给宿主细胞带来压力并激活反调节机制。内质网（ER）中的蛋白质过载会导致ER应激并触发未折叠蛋白反应，这反过来又会上调蛋白质折叠并增加内质网中蛋白质的降解。先前的研究表明，巨细胞病毒通过降解传感器分子IRE1来干扰未折叠蛋白反应。在此我们证明了巨细胞病毒M50蛋白如何利用ER相关降解机制来处理IRE1。IRE1的降解抑制了未折叠蛋白反应，并帮助病毒增加自身蛋白质的合成和病毒后代的产生。
